Get trace_regblock_size from metadata instead of event

In ctf trace, for each 'R' block, we save it as a "register" event,
as defined below in metadata.

event {
        name = "register";
        id = 0;
        fields := struct {
                ascii contents[440];
        };
}

Nowadays, we initialize trace_regblock_size by getting the length of
"contents" from a "register" event.  However, 'R' block may not exist
in traceframe, as a result, "register" event doesn't exist in trace file
and trace_regblock_size isn't set.

This patch changes to get trace_regblock_size from metadata (or declaration)
which always exists.

gdb:

2014-04-19  Yao Qi  <yao@codesourcery.com>

	* ctf.c (handle_id): New static variable.
	(ctf_open_dir): Get handle_id from bt_context_add_trace return
	value.  Get the declaration of event "register" and get length
	of field "contents".